Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-07-08
2003-03-11
Rogers, Scott (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S003060, C358S003130, C358S003190, C358S003200, C358S536000
Reexamination Certificate
active
06532082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital halftoning for printing. In particular this invention relates to a method for producing halftone printing plates containing microscopic ink repelling areas.
2. Description of the Prior Art
Printing can be done using different printing processes. The most important processes currently used are offset, gravure and flexography. Each of these processes has its advantages and disadvantages, and these processes compete with each other in price, quality and flexibility. Currently, offset is the most important process, certainly when the printing substrate is paper.
The offset process produces a printed result on a substrate by bringing over ink selectively from an ink source to an offset plate, then to an offset blanket and finally onto the substrate. Whether the final substrate will be inked (printed) or not in a particular location is determined by whether or not a corresponding zone on the printing plate will be inked.
Offset plates are flat plates which, when ready for printing, contain both ink repelling and ink receptive zones. The ink receptive zones receive ink and transfer the ink to an offset blanket, which in turn transfers the ink onto a substrate which may be paper, carton board, or other media. In traditional offset printing, the ink repelling zones accept water, and once humid, do not accept ink, so that the corresponding areas on the substrate are left blank. Waterless offset printing methods also are known in the prior art.
Whether traditional or waterless offset is used, the location of the ink receptive zones typically is determined by digital artwork, which may include linework (also called line art), continuous-tone, “contone,” (“CT”) images, or a combination, and the way the artwork is screened. Screening is the process of creating the illusion of a continuous-tone image on a device which can only reproduce two output values—ink or no ink in the case of an offset printing press. The continuous-tone illusion is created by the placement of small printing elements, known as halftone dots. When screening is done through digital operations, it is sometimes called digital halftoning. Thus, “digital halftoning” shall refer to such digital screening herein.
The way the digital artwork determines the plate is straightforward in areas where the digital artwork is blank (no ink needed, 0% fill) or fully filled (100% fill). Where it is blank, the plate will be fully ink repelling and where it is fully filled, the plate will normally be fully ink receptive. In all other cases, screening typically is used to obtain the desired appearance on the final printed result.
In some of the prior art, traditionally, the screening process places halftone dots centered on a square grid determined by a fixed angle and a fixed periodicity. The periodicity sometimes is expressed as the ruling, which is frequently expressed as a number of dots per inch (dpi), and sometimes also as the number of lines per inch (lpi) since the dots are usually arranged in lines. Density variations are achieved by changing the size of the halftone dots while the number of halftone dots in a given area is the same for high densities as for low densities. For example, 10% and 50% density regions have the same number of halftone dots but the area of a 50% dot is 5 times larger than the area of a 10% dot. This kind of screen is known as the “classical screen.” The technology of classical screens is very popular and covers currently more than 95% of the printing business. Variation of the size of the halftone dots in digital halftoning for classical screens is done by varying the number of black pixels making the halftone dot.
FIG. 2
shows a gradation of a classical screen
201
and is thus labeled “Prior Art.”
In classical screening, whether done digitally or not, the angles and periodicity of the square grid of dots are determined by technical considerations. For example, color printing involves printing in several colorants (inks), and several plates are prepared, one for each ink. These are called color separations. Printing is achieved by overlaying the different colors. Different angles are used for each separation to avoid moiré when overlaying the different printing colors. Rulings are determined by the process parameters such as substrate quality, ink viscosity, press characteristics and so on. Generally, one tries to use a ruling as high as possible to achieve the best image sharpness, taking care, however, not to exaggerate the ruling in order to avoid technical problems on the press.
Producing good quality printed material with an offset process is not an easy task. As is known in the printing industry, there are many problems which affect the quality of the final printed result, some even making printing impossible. The following is an incomplete list of some of the important problems.
substrate feeding problems causing press stops, bad quality or excessive waste of substrate;
density variations (e.g., too dark, too light, moiré patterns, etc.);
drying problems (e.g., ink dries too early or too late);
ink water balance (e.g., too much water, not enough water, etc.); and
inking problems (e.g., too much ink, not enough ink, ink on wrong places, etc.).
All such problems have in common that they diminish the quality of the produced printed material or the productivity of the press or increase the waste of substrate.
One of the choices one has to make for an offset printing is selecting the ink. On an even, high quality substrate, one can choose a strong, concentrated ink, with a brilliant aspect. By nature, strong inks typically are very sticky (high tack) and pull on the substrate during printing. When used on lower quality substrates, which are weaker or may have a rough surface, strong inks tend to pull particles away from the surface of the paper. Moreover, forces which pull on the substrate itself can cause shifts of the substrate in the press, and can lead to register problems or dot slur and even dot doubling. In an extreme case, the substrate can remain sticking on the press blanket and cause a paper jam, which unavoidably causes a press stop. On a web press, the web can beat due to the substrate which first sticks to the blanket, then gets pulled off by the web. This causes a dirty printed result. The operator typically solves this problem by making the inks “softer”, through the use of additions to the ink or the water, or by choosing softer inks supplied by ink manufacturers. As a result, dot gain can increase and the final result will be less brilliant. Moreover, the press will typically be run with too much water. The printing results will be pale. There is also an increased risk that moisture may enter the ink rolling system. When this happens, ink and water make a mixture which introduces quality problems and causes the ink rolling system to require more frequent cleaning.
The printing industry has been searching for solutions to these problems for a long time. Over the years, printing presses have improved in quality and degree of control, new ink types have been developed, and substrate manufacturers have delivered higher quality substrates and products of more consistent quality.
An important consideration in printing is the printing plate itself. Depending on the nature of the image on the printing plate, the above mentioned problems exist to a greater or lesser extent. There is thus an economic incentive to define imaging methods that produce printing plates which guarantee less troublesome printing.
As would be clear to those of ordinary skill in the art, a film often is used to make an offset plate, and whatever characteristics are required in the offset plate, corresponding characteristics also would be required in the film, the correspondence depending on whether the film is positive or negative. Thus, it is to be understood that the word “offset plate” when used hereunder means the offset plate and/or the corresponding film for making the offset plate for the case that such a film is used.
The
Esko-Graphics, N.V.
Inventek
Rogers Scott
Rosenfeld Dov
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